Hop acids, such as alpha-acids (principal analogues are humulone, cohumulone and adhumulone; also collectively named humulones), iso-alpha-acids (principal analogues are the cis- and trans-isomers of isohumulone, isocohumulone and isoadhumulone; also collectively known as isohumulones), beta-acids (or lupulones; mainly lupulone, colupulone and adlupulone), and derivatives of the aforementioned hop acids (for example reduced and oxidized derivatives), have a rather low solubility in aqueous media, especially in acidic aqueous conditions (Spetsig, Acta Chemica Scandinavica 9 (1955) 1421). At neutral pH and 298 K, alpha-acids have a solubility of about 5000 mg/L in water, while beta-acids have a solubility of only about 2 mg/L in water. During the wort boiling stage in the beer brewing process, typically at a pH value of about 5, the solubility is about 200 mg/L for the alpha-acids and about 10 mg/L for the beta-acids. Under the storage conditions of finished beer (pH 4 and 278 K), the solubility of alpha-acids is about 5 mg/L and <1 mg/L for the beta-acids, while the isomerized derivatives of the alpha-acids, i.e. the bitter iso-alpha-acids, have a solubility of about 120 mg/L under those conditions (Briggs et al., Brewing Science and Application (2004) 287, Woodhead Publishing Limited Cambridge England).
These variations in solubility in (acidic) aqueous media, with beta-acids exhibiting the lowest solubility, alpha-acids slightly higher, and the iso-alpha-acids the highest, are directly related to the pKa values of these hop acids. The beta-acids have pKa values around 6. The pKa values of the alpha-acids are around 5 (cohumulone 4.7, humulone 5.5 and adhumulone 5.7) and the iso-alpha-acids have pKa values of about 4. This implies that upon addition of iso-alpha-acids, and especially of alpha-acids and beta-acids, to an (acidic) aqueous medium with a pH about 5 (wort boiling conditions) or about 4 (finished beer conditions) a large fraction of the added hop acids will remain in the free acid form and will thus not readily dissolve in the acidic aqueous medium.
This low solubility of hop acids in acidic aqueous media has major disadvantages for their application in brewing (Meilgaard et al., Proceedings of the European Brewery Convention (1955) 109). As a consequence of their low solubility, typically only 50-55% of the alpha-acids (for example added as hop pellets or as a hop extract, produced by extraction of hop pellets using e.g. liquid or supercritical CO2) are extracted into and dissolved in the boiling wort medium, and only this dissolved alpha-acid fraction readily isomerizes to the desired, bitter iso-alpha-acids (Briggs et al., Brewing Science and Application (2004) 287, Woodhead Publishing Limited Cambridge England). After the wort boiling stage, the iso-alpha-acid yield is typically about 40 to 45%, which corresponds to an isomerization efficiency of about 80% for the dissolved alpha-acids. A higher fraction of extracted and dissolved alpha-acids in the boiling wort medium would thus positively affect the alpha-acid bittering utilization. Alpha-acid (bittering) utilization is defined as the fraction in terms of percentage of bitter iso-alpha-acids in the final beer versus the quantity of alpha-acids added during the brewing process. With further iso-alpha-acid losses after the wort boiling stage (because of lower solubility at beer pH, association to the yeast, adsorption to the filter, etc.), alpha-acid bittering utilizations for additions to the (boiling) wort (e.g. at 50 ppm alpha-acid addition levels) are typically in the 30 to 35% range.
Moreover, given the even lower solubility of beta-acids in the (boiling) wort, the actual use of their bacteriostatic potential in the brew kettle is even more limited as a result of an even lower fraction of dissolved beta-acids, with dissolved beta-acid fractions below 5% for 20 ppm beta-acid addition levels.
Low efficiencies are also observed when iso-alpha-acids in the free acid form, such as in the case of isomerized hop extracts, also known as isomerized kettle extracts, (produced, starting from a regular hop extract, by alpha-acid isomerisation in alkaline aqueous conditions followed by acidification of the process mixture to yield iso-alpha-acids in the free acid form as an organic layer, also containing beta-acids and hop oils), are added to the (boiling) wort, typically leading to iso-alpha-acid utilizations of 45 to 50% (e.g. at 40 ppm iso-alpha-acid addition levels), as a result of the limited extraction of the iso-alpha-acids from the isomerized kettle extract (IKE) phase into the acidic wort medium. Iso-alpha-acid (bittering) utilization is defined as the fraction in terms of percentage of bitter iso-alpha-acids in the final beer versus the quantity of iso-alpha-acids added during the brewing process.
Upon addition of reduced derivatives of iso-alpha-acids (such as tetrahydro-iso-alpha-acids, with a higher hydrophobicity and therefore lower solubility in aqueous media compared to the non-reduced iso-alpha-acids) in the free acid form, to the (boiling) wort, low utilizations (20-40%) are also obtained.
To improve the efficiency of extraction and dissolution of hop acids in (acidic) aqueous media and thus the brewing utilization of these hop acids, methods involving transformation of the hop acids to specific derivatives have been proposed. The primary focus has been on increasing the addition efficiency of iso-alpha-acids for post-fermentation bittering application.
Koller reported on the use of alkaline earth metal (e.g. Mg) salts of the iso-alpha-acids (e.g. in the form of finely ground particles) as beer bittering agents (Koller, Journal of the Institute of Brewing 75 (1969) 175). The same Mg salts of iso-alpha-acids can be found in isomerized hop pellets (hop pellets in which a >90% fraction of the alpha-acids is transformed into Magnesium-iso-alpha-acid salts or magnesium isohumulates), a hop product type widely applied for additions to (boiling) wort. These Mg isohumulates (added as isomerized hop pellets) however result in iso-alpha-acid utilizations comparable to or slightly lower than the utilizations obtained with iso-alpha-acids in the free acid form (as in an IKE), to say 40-45%, in the case of additions to the wort.
Alternatively, U.S. Pat. No. 3,532,504 (1970) reports on the alkali metal (e.g. K) salts of iso-alpha-acids as bittering agents, leading to increased iso-alpha-acid dissolution efficiencies and thus iso-alpha-acid utilizations in comparison with iso-alpha-acids added in free acid form. U.S. Pat. No. 3,949,092 (1976) also reports the use of potassium salts of iso-alpha-acids as beer flavoring agents. U.S. Pat. No. 5,015,491 (1991) informs on the production of such alkali metal isohumulates. PIKE, an isomerized hop extract mixed with a potassium compound, is also a beer bittering agent of the alkali metal-iso-alpha-acid salt type, however during the PIKE production undesirable alkaline degradation of the iso-alpha-acids can occur. In the brewing practice, it is known that these alkali metal isohumulates (as in PIKE) can lead to iso-alpha-acid utilizations above 50% for additions to the (boiling) wort, which represents a limited improvement compared to adding iso-alpha-acids in the free acid form. In GB 1,058,975 and 1,058,976, multi-step processes for the production of alkali metal salts of alpha-acids in powder form were presented.
Hudson and Rudin introduced ammonia-iso-alpha-acid complexes, prepared by passing ammonia in a methanolic solution of iso-alpha-acids (Hudson and Rudin, Journal of the Institute of Brewing 65 (1959) 416). U.S. Pat. No. 3,636,495 (1970) reports on a method for the preparation of granular ammonia-iso-alpha-acid complexes in an anhydrous solvent. The preparation of these ammonia-iso-alpha-acid complexes however involves the use of toxic and corrosive ammonia gas as well as hydrocarbon solvents. The iso-alpha-acid utilization obtained with these ammonia-iso-alpha-acid complexes was however below 40% for addition to the wort with a 50 ppm iso-alpha-acid addition level.
The above shows that in the prior art three principal methods were introduced to increase the extraction and dissolution efficiency of hop acids upon addition of those hop acids to (acidic) aqueous media during the brewing process, such as the (boiling) wort in the brew kettle or the brewed beverage stream prior to the final filtration. These methods are based on the formation of alkaline earth metal-hop acid salts, on the formation of alkali metal-hop acid salts or on the formation of ammonia-hop acid complexes. Only the alkali metal-hop acid salts lead to a marked improvement in terms of extraction efficiency into an (acidic) aqueous medium, such as wort, compared to the analogous hop acids in the free acid form.
These methods have however a number of limitations and drawbacks; there thus remains a need for an improved method for the extraction and the dissolution of hop acids in aqueous media, and in particular a need for a method to increase the utilization of hop acids in the brewing process.